Consumption of N2O and other N-cycle intermediates by Gemmatimonas aurantiaca strain T-27

Authors: Chee Sanford, Joanne; TIAN, D - University Of Illinois; SANFORD, R - University Of Illinois

Submitted to: Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2019
Publication Date: 12/1/2019
Citation: Chee Sanford, J.C., Tian, D., Sanford, R.A. 2019. Consumption of N2O and other N-cycle intermediates by Gemmatimonas aurantiaca strain T-27. Microbiology. 165:1345-1354. https://doi.org/10.1099/mic.0.000847.
DOI: https://doi.org/10.1099/mic.0.000847

Interpretive Summary: One of the most abundant groups of bacteria found in a wide variety of soils is the Gemmatimonadetes. Their elevated presence compared to many other better known bacteria in soils suggests this group is also functionally signficant, yet their functional diversity and role in important nutrient cycles, especially in agricultural soils, is not yet known. In a multi-year study at two Illinois field locations, we observed a high abundance of the nosZ gene encoding nitrous oxide reductase, the enzyme responsible for reducing the greenhouse gas nitrous oxide (N2O) to dintrogen (N2) belonging to Gemmatimonadates. In this study, we assessed the ability of N2O reduction by one cultivated member of this group, Gemmatimonas aurantiaca, and compared its ability to reduce N2O to the standard model of denitrification, where the activity takes place only under strictly O2-free conditions. We found that G. aurantiaca grows while reducing N2O under atmospheric O2 levels as well as reduce N2O under low (microaerophilic) O2 or strictly anaerobic conditions. This study is the first to show N2O reduction in a species of Gemmatimonadetes, but more importantly, that the activity occurs under oxic conditions, suggesting a significant sink for N2O may reside with these abundant members of the soil community even when soil remains unsaturated. The significance of these results changes our current views of bacterial denitrification, N2O reduction, and N-cycling, where an important soil bacterial population that has been understudied plays a potential large role in mitigating the greenhouse gas N2O.

Technical Abstract: Bacteria affiliated with the phylum Gemmatimonadetes occur at relatively high abundance levels in many soil microbial communities. In addition, soil metagenomes have revealed that the nitrous oxide reductase (nosZ) gene associated with this group is also common in soils, which is of interest since N2O is a greenhouse gas. NosZ catalyzes the terminal step in denitrification. Interestingly, the cultured representative of this group, Gemmatimonas aurantiaca has never been reported to grow anaerobically with N2O as the electron acceptor or under any denitrifying growth conditions even though it possesses a nosZ gene. Our experimental objective was to evaluate this organism’s ability to use N2O as an electron acceptor for growth. The ability to denitrify using nitrate or nitrite was also tested even though no nitrate reductase gene has been identified in the genome. Cultures started under anaerobic conditions on nitrate, nitrite or N2O failed to grow or show depletion of these substrates. Nitrate and nitrite also failed to be used even when cells were grown aerobically with the O2 allowed to deplete first. N2O reduction only commenced in the presence of O2 and continued to be depleted when refed to the culture. G. aurantiaca cells grown aerobically with N2O were diluted by 50% into fresh anaerobic, microaerobic (5% O2) or fully aerobic medium containing 1% N2O. After 48h, 36.8%, 30.5%, and 15.4 % of the N2O added was reduced in anaerobic, microaerobic and aerobic cultures, respectively. After 7 days, the microaerobic N2O reduction reached 81.6%, slightly greater than that observed anaerobically (79.3%) even though O2 was still detected in the headspace. Controls with no cells showed no loss of N2O. CO2 generation was observed under all conditions. Results show that G. aurantiaca does use N2O as an electron acceptor, however it does not grow as a typical denitrifier. Evidence also suggests that this aerobic organism may reduce N2O even in the presence of O2 and that only under atmospheric O2 conditions is the N2O reduction rate reduced. Our results demonstrate that N2O consumption may be possibly mediated in soil by members of the Gemmatimonadetes that are missing the typical denitrification pathway but could represent a significant sink for this greenhouse gas.